Cold insoluble globulin (CIG), a component of serum necessary for normal reticuloendothelial system function, opsonizes certain targets for enhanced removal by phagocytic cells. We have recently determined that CIG from citrated rat plasma significantly stimulates lymphoproliferation and enhances in vitro allograft responsiveness. The major objective of this proposal is to determine, in rats, the nature of the cell types which interact with CIG and the mechanisms by which CIG effects its immunoregulatory capability in vitro. In particular, emphasis is placed on the role of the macrophage in the CIG-lymphocyte interaction. Purified T and B lymphocytes and macrophages will be examined for their ability to bind CIG via specific receptors. Binding will be quantified with radiolabeled CIG and analyzed by Scatchard analysis. In addition, it will be determined whether CIG stabilizes the lymphocyte-macrophage interaction, thus suggesting a possible mechanism by which CIG promotes nonspecific lymphocyte proliferation. The macrophage requirement for the stimulatory activity of CIG will be examined directly by the response of macrophage-depleted leukocyte populations and by the repletion of these cultures with graded numbers of normal and CIG-"pulsed" macrophages. The possible immunoregulatory effect of CIG on various antigen-specific responses will also be determined by assaying the primary and secondary response to SRBC using both plaque assays and a radioimmunometric assay for total immunoglobulin and specific antibody. In addition, the effect of CIG on allograft responsiveness in the mixed lymphocyte reaction will also be determined. The direct effect of CIG on these responses will be determined by examining whether CIG or CIG in conjunction with macrophages activates a select population of regulatory lymphoid cells: helpers, amplifiers or suppressors.